concurrency-101.md

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author	Ali Ahmad
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Java Concurrency

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Recap

• Lambda's
• Streams

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Java Concurrency

• Threads
• Process
• Using Threads
• Daemon threads
• Thread LifeCycle

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Thread

• A thread is a single sequential flow of control within a program.

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Thread

• Single sequential flow of control
• Allow the program to split into simuntanously running task
• As a metaphor - A worker who is building a wall

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Process

Execution -> process

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Process

• Load program to memory
• Allocate the resources
• Execute the program

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Process

• A process is a manifestation of a run state of an application.

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Process

• Binary instructions loaded into memory
• Get access to resources like memory
• Resource is protected from other processes

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Process vs Threads

• A process can be single threaded or multi threaded
• A process can spawn multiple threads
• Unlike process, threads in a process can share the same memory and resources

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Process with single thread

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Process with multiple threads

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A Java Application

• A single process (JVM)
• Consists of various threads (Garbage collector, main, etc)
• Application thread - responsible for running the main method

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Threads Creation - The classic way

• Identify the piece of code, that you want to run in a separate thread
• Create a Runnable implementation for that task

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Threads Creation - The classic way

public class MyRunnable implements Runnable {

    @Override
    public void run() {
        MyTask.run();
    }
}

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Threads Creation - The classic way

Runnable myRunnable = () -> MyTask.run();

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Threads Creation - The classic way

• Create a thread and pass the runnable to it
• Start the thread

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Threads Creation - The classic way

Thread myThread = new Thread(myRunnable);

myThread.start();

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Demo

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How this works

• Thread class is just there, defining meta data for the thread
• JVM calls the underlying OS threading API

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When does a thread end?

• When the run method returns
• When exception is thrown

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Daemon Threads

• When does the application end?

• When you spawn threads, it ends when all the threads end

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Daemon Threads

• Daemon threads are threads that end when the main thread ends
• Do not want a thread to extend beyound the lifecycle of the application, create a daemon thread
• Daemon threads are created by calling setDaemon(true) on the thread object

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Daemon Threads

Thread myThread = new Thread(myRunnable);
myThread.setDaemon(true);
myThread.start();

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Thread Lifecycles

• New
• Runnable
• Blocked
• Waiting
• Timed Waiting
• Terminated

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Thread Lifecycles

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Thread States

• Running vs Runnable?

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